Remove monkey-patch of the Numeric class

This monkey-patch doesn't seem to be working with Zeitwerk, and we were
only using it in one place, so the easiest way to solve the problem is
to remove it.

Note that, in the process, we're changing the operation so `* 100`
appears before the division, so it's consistent with other places where
we do similar things (like the `supports_percentage` method in the
proposals helper).

app/helpers/votes_helper.rb

@@ -1,6 +1,6 @@
 module VotesHelper
   def debate_percentage_of_likes(debate)
-    debate.likes.percent_of(debate.total_votes)
+    (debate.likes.to_f * 100 / debate.total_votes).to_i
   end
 
   def votes_percentage(vote, debate)

app/models/debate.rb

@@ -1,5 +1,3 @@
-require "numeric"
-
 class Debate < ApplicationRecord
   include Flaggable
   include Taggable

lib/numeric.rb

@@ -1,5 +0,0 @@
-class Numeric
-  def percent_of(n)
-    (to_f / n * 100).to_i
-  end
-end